[chore][confmap] Add test for edge case with escaped default value (#14720)

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#### Description

Relates to
open-telemetry/opentelemetry-go-contrib#8342 (comment)

Author: mx-psi

confmap/internal/e2e/expand_test.go

@@ -49,6 +49,8 @@ func Test_EscapedEnvVars_NoDefaultScheme(t *testing.T) {
 			"key18": "here is 1 $",
 			"key19": "here are 2 $$",
 			"key20": "some expanded value came from nested expansion",
+			"key21": "default_value",
+			"key22": "${env:UNDEFINED_KEY:-${env:UNDEFINED_KEY}}",
 		},
 	}
 
@@ -97,6 +99,8 @@ func Test_EscapedEnvVars_DefaultScheme(t *testing.T) {
 			"key18": "here is 1 $",
 			"key19": "here are 2 $$",
 			"key20": "some expanded value came from nested expansion",
+			"key21": "default_value",
+			"key22": "${env:UNDEFINED_KEY:-${env:UNDEFINED_KEY}}",
 		},
 	}
 

confmap/internal/e2e/testdata/expand-escaped-env.yaml

@@ -39,3 +39,7 @@ test_map:
   key19: "${env:ENV_NESTED_DOLLARSIGN_ESCAPED}"
   # nested env var syntax is expanded
   key20: "${env:ENV_EXPAND_NESTED}"
+  # default value
+  key21: "${env:UNDEFINED_KEY:-default_value}"
+  # escaped default value
+  key22: "${env:UNDEFINED_KEY:-$${env:UNDEFINED_KEY}}"